As known, overhead electric lines represent by far the most widely used type of long-distance power line. They comprise a plurality of conductors stretching between support poles. Normally, especially for higher-voltage long-distance power lines, each line conductor consists of a bundle of cables or basic conductors, because that enables to increase transmissible power, limiting leakage and electromagnetic disturbance fields. The most commonly used bundles consist of two, three or four basic cables.
Evidently, the individual bundle conductors must be kept at the correct distance one from the other, preferably across the entire span between one support pole and the other. For such purpose, it is known to resort to spacers, constructed in various ways depending on the number of conductors making up the bundle. In the present specification, spacers for 4-cable bundles will be dealt with, i.e. spacers suited to maintain a position coherence between four cables belonging to a bundle of a conductor.
In static conditions, few spacers located across the length of the span (varying between 200 and 1000 meters long) are theoretically sufficient to keep the cables at the correct mutual distance, defining a series of wide subspans. However, the overhead transmission lines are exposed to varying atmospheric conditions which change the condition thereof and introduce external disturbance forces: typically, wind acts significantly on overhead transmission lines, affecting the dynamic behaviour thereof.
The wind action introduces three types of oscillations/vibrations on overhead transmission lines. Aeolian vibrations, due to whirl detachment, normally manifest at high frequency and low amplitude. Another oscillation mode is the one referred to as “galloping” which originates—with low frequency and high amplitude—within the same span (i.e. between two support poles) and leads to the cable bundles oscillating in the vertical plane. The galloping occurs in highly specific environmental conditions (typically when there is an ice deposit around the cables). Finally there are subspan oscillations, which manifest within the individual subspan mutually separated by the spacers and are due to an aerodynamic interaction (wake effect) between the windward cables and the leeward ones.
In the following it will be substantially dealt with subspan oscillations. This phenomenon proved particularly evident on 4-cable bundles, where typical conditions exist where finds himself having a pair of windward cables and a pair of leeward cables, which produce important wake effects. These oscillations, also referred to as ‘subspan’, may lead to collisions between the bundle sub-conductors with remarkable stresses on the conductors, at the spacer damper clamps, and with the possible resulting cable breakage.
All the 4-cable spacer dampers suggested so far in the prior art provide a central quadrilateral framework, at the vertexes of which there are arranged fastening means, supported by respective small arms, at the ends of which the cables of the conductive bundle are tightened. Typically the structure is symmetrical, so as to maintain the four cables at the vertexes of a quadrilateral, mutually well spaced apart and with an equally distributed static load.
In order to face subspan oscillations, over time various devices have been suggested.
Traditionally, the cable-fastening arms are mounted on the quadrilateral framework through dampening hinges, which already introduces a dampening effect on oscillations/vibrations. Spacer dampers of this type are described, for example, in EP0244624, U.S. Pat. No. 4,554,403 and U.S. Pat. No. 4,188,502.
Also in FIG. 1 herewith enclosed there is shown, in elevation front view, a spaces damper of the prior art. As can be noticed, the spacer damper consists of a quadrilateral framework 1, at the vertexes of which four identical arms 2a-2c are hinged, at the distal ends of which there are fastened, with known-type clamps, the conductive cables (not shown). Each hinge 3a-3c constraining framework 1 to arms 2a-2c has a configuration suited to support the clamps in the desired position and to dampen any oscillations. In particular, as can be noticed in FIG. 1, the clamps are supported so that the cables are virtually at the same height as the centre of rotation of hinges 3a-3c (i.e. the fastening point of the cable and the rotation axis of the hinge are on a substantially horizontal line). This design is considered the most suitable in this sector because, in addition to producing a symmetrical load of the stresses, it is the one which allows to face best aeolian vibrations. As a matter of fact, said aeolian vibrations are triggered typically in the vertical plane (coinciding with the y axis of FIG. 1) and hence it is useful that they find a good lever arm defined by arms 2a-2c with respect to the axes of rotation of hinges 3a-2c so that the dampening system operates adequately. FIG. 2 shows a geometric diagram by which the space damper of FIG. 1 is schematised for numerical dynamic simulations. As can be detected, the nearly horizontal attitude of the arms and the need to space the cables apart as far as possible makes the central framework substantially rectangular and of significant size (the height thereof is nearly identical to the vertical distance between the cables).
Another effective measure is certainly that of increasing the number of the spacer dampers on the transmission line, in order to reduce the extension of subspans and hence improve the behaviour of the bundle upon aerodynamic forces. When the conductive cables are particularly light and the weather conditions are particularly severe (high temperatures, reduced tension on the cables and frequent and intense winds), the subspan oscillation is a predominant problem and it may be necessary to shorten the subspans down to about 40-45 meters. This is particularly undesired—as may be guessed—because it affects costs, both of the material and of installation and maintenance.
Alternatively or additionally, it has been suggested to apply to the subspan line suitable dampening devices, provided with counterweight, which act especially on torsional normal modes. This solution, however, affects costs, too.